Effect of Local Periodic Disturbance on Mixing Layer Downstream of Two-Dimensional Jet (Effect of Amplitude and Frequency of the Disturbance)

Abstract

The laminar-turbulent transition of a mixing layer induced by oscillating flat plates at the exit of a two-dimensional nozzle was experimentally investigated. The mixing layer was formed between the jet, which issued from the nozzle, and the surrounding quiescent fluid. The plates oscillated vertically in relation to the jet. The oscillation frequency was two orders of magnitude smaller than the fundamental frequency of the fluctuating velocity. The oscillating amplitude was double that of a previous report. The oscillating frequency was pseudo-steady and the same as in the previous report, at 5 Hz, and unsteady at 15 Hz. Mean and fluctuating velocity components in the streamwise and normal directions were measured by hot-wire anemometers. As the flow proceeds downstream, the mixing layer entrains the surrounding quiescent fluid. This entrainment is affected by the oscillating frequency rather than the oscillating amplitude. The variation of the streamwise velocity with the oscillating phase is much affected by the oscillating amplitude rather than the oscillating frequency. On the other hand, the variation of the normal velocity is affected by both oscillating amplitude and frequency. For an irregularly fluctuating velocity, a linear region where the velocity grows exponentially first exists. The spatial growth rate in this region is the same between the streamwise and normal components, and does not differ with various oscillating amplitudes and frequencies.